feat: add emulator profiles and cross-reference engine (tier 2)

New two-tier architecture:
- Tier 1: Platform configs (what the UI checks) - unchanged
- Tier 2: Emulator profiles (what the code actually loads)

11 emulator profiles from source code analysis:
  cemu, citra, dolphin, duckstation, flycast,
  melonds, pcsx2, ppsspp, rpcs3, vita3k, xemu

Each profile documents every file the emulator loads with
source code references (file:line), hashes, and notes.

New scripts/cross_reference.py computes gaps between what
platforms declare and what emulators need.

Current gap: 200 undeclared files, 24 already in repo.
DuckStation alone recognizes 105 PS1/PS2 BIOS variants.

generate_pack.py gains --include-extras flag (future use).
_registry.yml maps platforms to their emulators.

schemas/emulator.schema.json

@@ -0,0 +1,35 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "Emulator BIOS Profile",
+  "type": "object",
+  "required": ["emulator", "source", "systems", "files"],
+  "properties": {
+    "emulator": { "type": "string" },
+    "type": {
+      "type": "string",
+      "enum": ["standalone", "libretro", "standalone + libretro"]
+    },
+    "source": { "type": "string", "format": "uri" },
+    "systems": {
+      "type": "array",
+      "items": { "type": "string" }
+    },
+    "files": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "required": ["name"],
+        "properties": {
+          "name": { "type": "string" },
+          "region": { "type": "string" },
+          "required": { "type": "boolean", "default": false },
+          "md5": { "type": "string", "pattern": "^[a-fA-F0-9]{32}$" },
+          "sha1": { "type": "string", "pattern": "^[a-f0-9]{40}$" },
+          "size": { "type": "integer" },
+          "source_ref": { "type": "string" },
+          "note": { "type": "string" }
+        }
+      }
+    }
+  }
+}